1. Context
Concentrated solar power (CSP) systems are ones that concentrate incoming solar light before converting it into useful power. The conversion itself can be photovoltaic or thermal, but the common theme is that it is cheaper to collect the light over a large area and into a small power conversion unit (PCU) than it is to build a large power converter.
There are several methods to concentrate solar power, including lenses, sun-tracking parabolic dish reflectors that position the PCU at the focus of a paraboloid, and central tower systems in which a large number of principally flat tracking mirrors direct the sun onto the top of a tower where the PCU is housed.
In the case of dish reflectors, the PCU is part of the moving structure since it has to be kept at the focal point of the dish as it tracks the sun. Additionally, since dish reflectors are typically large, they themselves are comprised of smaller reflectors held rigidly together to form the complete optical surface. Typically, these smaller reflectors need to be aligned relative to the dish structure during assembly in the field. Dish reflectors can use photovoltaic (PV) PCUs, Stirling engines, turbines, or heat collectors and steam generators.
2. State of the Art
FIG. 1 shows a conventional solar thermal dish design (manufactured by Stirling Energy Systems of Arizona) containing a thermal PCU [10] connected to a dish through a boom [11] (or balance beam), which connects to pivot actuation machinery [12] that is mounted on a pole (also called a pedestal) [13]. The dish and PCU [10] are roughly balanced via the boom [11] with respect to the pedestal [13], reducing the gravity loads on the actuation machinery [12]. The dish is comprised of a carrier truss [14] and reflector tiles [15]. Because of the balance-beam design, such dishes must have a slice [16] cut into them to prevent the pedestal [13] from hitting the dish when it points upwards. The shape of the reflector surface of the dish (made up of the sum of the reflector surfaces of the tiles [15]) is approximately a paraboloid (a parabolic arc revolved around its optical axis) and the aperture of the PCU [10] is located at the focal point of the paraboloid. The pivot actuation machinery [12] is controlled by a sun-tracker that keeps the optical axis of the dish pointed at the sun.
The reflector tiles [15] themselves are made from thin glass which is warped elastically over a metallic shell and bonded to it. In other systems, thick glass is hot-formed and plastically deformed into the desired shape. In other systems, the thin glass is replaced with a thin Aluminum or Steel sheet with a reflective coating.
FIG. 2 shows a schematic of a Stirling engine PCU, which is a type of a thermal engine. (In FIG. 1, the PCU was denoted as item 10). Like all thermal engines, this PCU has a hot side [20] and a cold side [21]. The hot side [20] is illuminated by the light reflected from the dish and is traditionally mounted facing it and closest to it. The cold side [21] is therefore traditionally mounted further away from the dish, towards the sun. The cold side [21] is connected to a heat exchanger [22] which rejects heat into the environment and keeps its temperature from rising. An electric generator [23] is powered by the engine and mechanically coupled to it. In most thermal designs, both the heat exchanger [22] and the generator [23] are part of the PCU package [10]. It is the high weight of thermal PCUs that traditionally dictates the balance-beam design for the dish.
FIG. 3 shows a conventional dish-based photovoltaic system (manufactured by Solar Systems of Australia). Since photovoltaic PCUs are lighter than thermal PCUs, the system does not use a balance beam, and instead the PCU [30] is connected to the dish via a focal support structure [31]. The dish, in turn, is directly connected to the actuation machinery [hidden] and pedestal [32]. The dish is comprised of a carrier truss [33] and reflective tiles [34], same as in the solar thermal dish and the shape of the optical surface of the primary dish is similarly a piece-wise segmented paraboloid. In a PV system, there is no generator, but there is still a need for a heat exchanger, since the PV cells need to be kept cold. In many PV dishes, the heat exchanger is located on the ground and coolant is piped between it and the PCU [30].
The optical area of both dishes shown above is about 100 m2. In the PV dish, the concentration factor is about 1000, the area of the aperture of the receiver is 0.1 m2, and it is built from approximately 1000 PV cells, each only 1 cm on a side, arranged in a “dense array” roughly 30 cm across (shown schematically in FIG. 4b, 44). Since PV cells produce low voltage (˜3V), and since the output voltage of the dish must be high (100-600 V) to keep the current manageable, many of the cells need to be wired in series, a process known as “stringing”. Cells on the same string must produce the same amount of current or else the efficiency of the string drops due cell current mismatch, known as a “stringing losses”.
FIG. 4a shows a single photovoltaic cell. The cell has an active area [41] covered with thin conductive lines known as the collection grid [42] that leads to two side contacts [43] commonly known as bus bars. The two bus bars [43] correspond to the “plus” side of the photovoltaic junction, and the back surface of the cell corresponds to the “minus” side. The grid lines [42] are created by metallic deposition and are made tall and thin to minimize shading, but still produce significant shading for light that is arriving at a shallow angle relative to the front surface of the cell. Cells designed for high concentration are typically made by depositing multiple layers of semiconductor materials on a Germanium substrate, but other technologies are equally relevant to this invention. Such “multi-junction” cells are made by companies such as Spectrolab, Emcore, and Solar Junction.
In traditional dish systems, the carrier truss is non-adjustable and is assembled in the field to the best practical precision. The truss is then placed on top of the pedestal, and the reflector tiles are assembled onto it using adjustable mating mechanisms—typically three adjustment screws at the back of each tile that connect to three points on the truss. At this stage, using an optical reference (e.g. pointing at the moon, or a laser system that bounces off of the reflector tiles) the orientation of each reflector tile is adjusted by turning the screws until it is properly aligned relative to the focal point.